1. Technical Field
The present invention relates in general to image processing, and in particular to digital image processing. Still more particularly, the present invention relates to a system and method for generating a variably scaled digital image.
2. Description of the Related Art
In a number of business and personal applications, it has become desirable to generate a variably scaled digital image in order to produce a display which may be smaller or larger than the original digital image. A number of applications exist where a digital image needs to be reduced or enlarged, due to the size of a display or as a result of the need or desire to place more than one image on a display at one time.
A simple technique utilized to reduce an image is the deletion of picture elements, known in the art as pixels or pels. The deletion of pels is accomplished by typically selecting every Nth pel in the X direction of the original image, and every Mth pel in the Y direction of the original image, and placing the pels in the reduced image while discarding all other pels to obtain a reduction in the size of the original image. An example of this technique is disclosed in U.S. Pat. No. 4,412,252, Moore et al., entitled Image Reduction System, issued Oct. 25, 1983.
A disadvantage of this technique, however, is that the information contained in the discarded pels is lost completely. Images containing linear features, like maps or text, experience an annoying visual effect when processed with this technique.
One way to mitigate this effect is to apply some filtering function that computes a local weighted average on the input pels within a specified neighborhood around the position of the output pel. The value of the output pel is then obtained as a contribution of the corresponding input neighbor pels. This technique has been applied by Tabata et al., High Speed Image Scaling for Integrated Document Management, ACM 2nd Conference on Office Information Systems, Toronto (June 1984), using a 2 by 2 neighborhood weighting window on the input image.
This approach, however, encounters the same disadvantage discussed above when the reduction factor is over 2. Some of the input pels are not included in the computation of any output pel, and the information contained in the discarded pels is lost completely.
Another technique utilized to generate a variably scaled digital image is to employ multiplication and/or division operations in the method of scaling an image. Although implementing multiplication and division operations in hardware improves the speed of the operations, those skilled in the art will recognize that multiplication and division operations require complex circuitry. The disadvantage to this technique is that the complex circuitry is very expensive.
U.S. Pat. No. 4,725,892, Suzuki et al., entitled Method and Apparatus for Enlarging or Reducing an Image on the Basis of Precalculated Reference Pel Contribution Rate Data and Two-Dimensional Reference Window Shifting Amount Data Stored in a Table Memory, issued Feb. 16, 1988, teaches a method for reducing or enlarging an image which utilizes a reference window. The reference window determines the pels which are included in a particular scaling step. During the reduction of an image, the reference window shifts its location for each output of a pel. When an image is enlarged, however, the reference window does not always shift a position for each pel output. This is due to the fact that the spacing between estimated pels becomes smaller as the enlargement ratio becomes higher. The failure to shift position causes one or more pels to be included in a scaling step more than once. Including one or more pels in a scaling step more than once means more power is spent on reducing an image. More operations are required to reduce the image, which eventually translates into increased costs.
Therefore, it would be desirable to provide a method and system for efficiently generating a variably scaled digital image which is less expensive and which generates a high quality digital image.